1. Field of the Invention
The present invention relates to sensor module, a correction method executed in the sensor module, and a mobile object including the sensor module.
2. Discussion of the Background
Recently, it has become common for mobile objects such as a car, a cellular phone, etc. to include an azimuth measurement device that detects terrestrial magnetism and that searches for a moving direction of the mobile object for a navigation system.
An azimuth measurement device may not always provide accurate information and may indicate a wrong direction since output signals of a magnetic sensor generally include offsets caused by extraneous magnetic fields other than terrestrial magnetism. Therefore, it is beneficial to detect such an offset so that the azimuth can be accurately determined.
Japanese Patent No. 1422942 discloses an output signal correction device for an azimuth measurement device that determines a traveling direction of a vehicle or so on. Japanese Patent No. 1660613 discloses a magnetic vector detecting method for eliminating an influence caused by a measurement device or a device mounted on a measurement device being magnetized, and that reference addresses a measurement device having an offset error. Japanese Laid-Open Patent Application No. 2000-131068 discloses an electric azimuth measurement device that can adjust a correction value that reduces an influence of an azimuth measurement device being magnetized (by other than just terrestrial magnetism), to match a maximum value or minimum value of each element of terrestrial magnetism.
Japanese Laid-Open Patent Application No. 2002-48551 discloses an adjusting method for an electric azimuth measurement device using a magnetism generating device. Japanese Laid-Open Patent Application No. H6-174472 discloses an azimuth measurement device that can demagnetize a magnetic sensor part by adding a magnetic field to the magnetic sensor part using a coil. Japanese Laid-Open Patent Application No. H6-249663 discloses an azimuth measurement device that can provide a warning in a case of judging that a signal input from a terrestrial magnetism sensor to a calculating part is outside of a predetermined range, and when the signal is outside of the predetermined range for a time exceeding a predetermined time. That azimuth measurement device can also change the predetermined time according to a quantity for a gap between the signal and an upper limit of the predetermined range or a lower limit.
Japanese Laid-Open Patent Application No. 2005-195376 discloses an azimuth angle measurement method that sets a coordinate point as a criterion point, which is at a position that minimizes a fluctuation of distance between the coordinate point and output data from a terrestrial magnetism part. That azimuth measurement device cancels the criterion point in the case the diameter of a circle, whose center is the criterion point and whose circumference is formed by the data, is outside of a predetermined range.
Japanese Laid-Open Patent Application No. 2006-47299 discloses a correction method that estimates offset measurement values from measurement data of a terrestrial magnetism sensor, calculates each distance between measurement data and the estimated offset measurement value on a coordinate, calculates an average of each distance, calculates root-mean-square deviation based on the each distance and the average, and judges the effectiveness of an offset measurement value based on the root-mean-square.
Japanese Patent No. 2538738 discloses an azimuth measurement device for a vehicle comparing an output value from a terrestrial magnetism sensor one after another and a magnetization intensity at that time (a diameter of a circle formed by output data), and that corrects a center point of the circle on a coordinate accordingly for minimizing an error based on the compared result. That method postulates that the circle formed by the output data is essentially a perfect circle.
Japanese Laid-Open Patent Application No. H7-151842 discloses a magnetism detection device that corrects an induced magnetic field caused by ferromagnetism utilizing a magnetic field by a permanent magnet mounted on an electric device. Japanese Laid-Open Patent Application No. 2003-167039 discloses a magnetic sensor and an azimuth measurement device with the magnetic sensor using a thin-film magnetic resistance effect element. Japanese Laid-Open Patent Application No. 2003-8101 discloses a tunneling magnetic resistance effect element and an azimuth measurement device with the element.